Prevention of crevice coking in heat exchangers

ABSTRACT

In a shell-and-tube sheet exchanger, a masking blanket of steam is provided in the space between the outlet conduit for the shell fluid and the tube sheet, at one end of the heat exchanger. This steam blanket serves as a barrier, to prevent the hot hydrocarbon fluid which is flowing through the shell from coming into contact with the tube sheet at this end of the exchanger.

United States Patent Jerry M. LaRue;

Bartlett A. Lloyd, both of Tulsa, Okla. 860,972

Sept. 25, 1969 Oct. 5, 1971 Sun Oil Company Philadelphia, Pa.

Inventors Appl. No. Filed Patented Assignee PREVENTION OF CREVICE COKING IN HEAT EXCHANGERS 1 1 Claims, 3 Drawing Figs.

US. Cl 165/134, 208/48 Int. Cl F281 19/00 Field of Search 165/134, 161; 23/277; 208/48 References Cited UNITED STATES PATENTS 5/1923 Jones 165/161 3,356,135 12/1967 Sayre 165/134 X 3,400,754 9/1968 Barbo et a]. 165/134 X 3,409,407 1 1/1968 Loeffler et a]. 165/ l 34 X 2,064,708 12/1936 Wilson 208/48 3,527,832 9/1970 Pamphilis et a1 208/48 X Primary Examiner-Frederick L. Matteson Assistant ExaminerThe0phil W. Streule Attorneys-George L. Church, Donald R. Johnson, Wilmer E.

McCorquodale, Jr. and Frank A. Rechif ABSTRACT: In a shell-and-tube sheet exchanger, a masking blanket of steam is provided in the space between the outlet conduit for the shell fluid and the tube sheet, at one end of the heat exchanger. This steam blanket serves as a barrier, to prevent the hot hydrocarbon fluid which is flowing through the shell from coming into contact with the tube sheet at this end of the exchanger.

PATENTED 0m 5|s1| 3610.328

JERRY M. LA BARTLETT .LLOYD/ BY QNJI PREVENTION OF CREVICE COKING IN HEAT EXCHANGERS This invention relates to heat exchangers of the shell-andtube type, and more particularly to exchangers of this type used to heat e.g. vaporize) a hydrocarbon material.

In heat exchangers used in high temperature hydrocarbon service, a tube failure phenomenon has presented itself, the tube failures eventually reaching such proportions that the exchanger must be shut down and the tube bundle retubed. This shutdown and the retubing operation is expensive, taking into account both loss of profits during shutdown and the actual cost of the repairs. The failure of the tubes is due to pinching off of the same within the hot tube sheet and in the immediate proximity of the backside of the tube sheet.

In shell-and-tube heat exchangers, there is ordinarily a' crevice around the outside of each tube on the backside of the tube sheet, since the expansion of the tube to the tube sheet is terminated a short distance (such as )fiinches or more) inside of the backface of the tube sheet to avoid cutting the tube when rolling it in. It has been determined that, in high temperature hydrocarbon service, the pinching off" of the tubes is due to a progressive accumulation of coke, formed due to vapor and/or liquid phase cracking of the hydrocarbon, which gets into the crevices around the tubes, in the hot tube sheet. The coke, being porous, will soak up more hydrocarbon, thereby forming a larger layer of coke. The coke produces a stress on the tubes and eventually pinches them off, the tubing material yielding since it is at a high temperature and its stress value is consequently greatly decreased.

It might be thought that a solution would be to roll the tubes completely, the full thickness of the tubes sheet. Although mechanically a tight pressure seal, that shows no crevice with a good quality roll, results, theoretically the crevice exists. Coke exerts tremendous forces during buildup, thus working or prying its way into any opening. In fact, the forces produced might by analogy be compared to those produced during the transformation of water into ice in the interstices of rock formations, which cause deterioration of the rock.

In addition, rolling the full thickness of the tubesheet presents the possibility of cutting the tube on the back edge of the tube sheet.

Crevice coking being the cause of the tube failures, a solution would be to eliminate the possibility of the hydrocarbon getting into the crevices. One way in which this could be done would be to weld the tubes to the backside of the tube sheet, thereby in effect eliminating the crevices. However, such hubwelded tube bundles are very costly.

An object of this invention is to provide a novel heat exchanger construction.

Another object is to provide a structure for the prevention of crevice coking in heat exchangers.

A further object is to provide a simple and effective arrangement, in a heat exchanger, for preventing the hot shell fluid from coming into contact with the crevices in the-tube sheet.

The objects of this invention are accomplished, briefly, in the following manner: In a shell-and-tube heat exchanger positioned with its longitudinal axis extending vertically, a baffle is mounted immediately adjacent to the shell outlet conduit (at the hot shell fluid end of the exchanger), to provide a chamber in the space between this conduit and the adjacent tube sheet. Steam is injected into this chamber to provide a masking blanket of steam therein, the pressure in this chamber being maintained somewhat in excess of the shell fluid pressure. A detailed description of the invention follows, taken in conjunction with the accompanying drawing, wherein:

FIG. I is a schematic representation of a heat exchangers as used in a process;

FIG. 2 is a longitudinal sectional view of the upper end of the heat exchanger of FIG. 1, showing the internal construction; and

FIG. 3 is a view, drawn on an enlarged scale, of a detail. Refer first to FIG. 1, which illustrates, by way of example, a heat exchanger as used in a process involving high temperature hydrocarbon service. A heat exchanger 1 of the shell-andtube type (certain details of the internal construction of which will be described hereinafter) is positioned withits longitudinal axis extending generally vertically. This heat exchanger is one of the pieces of apparatus used in a styrene production process, and may be termed a styrene reactor feed-effluent exchanger, or a reactor effluent-EB (for ethylbenzene) vaporizer. The hot effluent from the reactor, flowing through the tubes of the exchanger, is used to heat (vaporize) the EB feed for the reactor, the feed flowing through the shell of the exchanger. Thus, the cold" EB feed coming from a feed tank (not shown) at a temperature of about F is fed into the lower end of the shell of exchanger 1 by way of an inlet 2 and flows upwardly through the shell, leaving the upper end of the exchanger by way of an outlet conduit indicated at 3. The EB feed leaving the exchanger 1 has a temperature of about 1000 F. and is in the form of a vapor. This vaporized feed, by way of example, may be fed to a reactor (not shown) which produces styrene.

Hot effluent, derived from the same aforesaid reactor, is fed at about 1070 F. by way of an inlet coupling 4 to a header 5 at the upper end of exchanger 1. This styrene reactor effluent flows downwardly. through the tubes in exchanger 1, counter to the flow of feed upwardly through the exchanger shell. The tube fluid (reactor eflluent) gives up heat to the shell fluid (EB feed) in the exchanger 1, and leaves the exchanger, by way of the bottom tube coupling 6, as cool reactor effluent (at a temperature of about 550 F.

It may be pointed out here that the upper end of the exchanger 1, at the shell outlet conduit 3, is the "hot end for the shell fluid.

Refer now to FIG. 2, which is a longitudinal section through the upper end of the heat exchanger 1. The lower end of header 5, which provides a chamber for receiving the tube fluid, is secured to the upper end of the shell 6 by means of a flange 7 which is bolted to a similar flange 8 provided on the upper end of the shell. A flanged cover plate 9 is bolted to the flanged upper end of header 5. A tube side inlet pipe 4 is sealed through the wall of header 5, the hot reactor effluent flowing through this pipe into the interior of header 5.

A radially outer annular area of a disklike stationary tube sheet 10 is sandwiched between flanges 7 and 8. A plurality of vertically extending tubes 11 (only one of which is illustrated in FIG. 2, in order to simplicity the drawings) pass through respective longitudinally extending holes 17 provided in the tube sheet, and the upper ends of these tubes are secured in and sealed into the front face or front side 10a of the tube sheet by being rolled over into their respective holes. The rolling over of the tubes 11 does not normally extend through the full thickness of the tube sheet 10, so that on the backside 10b of the tube sheet a crevice 18 (see FIG. 3) remains surrounding each tube, between the tube OD. and the ID. of its respective hole 17 in the tube sheet. By way of example, these crevices 18 may be from flainch to %inch in depth, measured from the backside 10b of the tube sheet. At this juncture, it is pointed out that the tubes 11 open into the space within header 5, so that the tube fluid supplied by way of the tube side inlet coupling 4 to the header enters the upper ends of these tubes and travels downwardly therein through heat exchanger l.

The shell side outlet conduit or pipe 3, through which the hot (vaporized EB feed leaves the exchanger 1, is sealed through the wall of shell 6, adjacent to but spaced somewhat below the tube sheet backside 105. This leaves a space between the outlet conduit or nozzle 3 and the backside 10b of the stationary tubesheet, into which (in a conventional heat exchanger) the hot hydrocarbon would enter, and would thus find its way into the crevices 18 around the tubes 11 (since these crevices open into the backside of the tube sheet). This space would be essentially stagnant, since the shell fluid, flowing upwardly from below the conduit 3, travels in the direction of the arrows 12 into the conduit 3, rather than up into this space.

The hot hydrocarbon (EB feed), mentioned, present in the crevices 18 around tubes 11, would produce coke therein, resulting in the eventual pinching off of the tubes 11, as previously described.

Even if the crevices were eliminated (which is a very expensive type of construction), there would still remain the possibility of shell failure or swelling outward, due to coke buildup in the stagnant space between conduit 3 and the stationary tube sheet 10.

According to this invention, the stagnant space previously mentioned (where coking occurs) is entirely eliminated, or, to put this another way, the hot hydrocarbon shell fluid is prevented from coming into contact with the crevices 18 in the backside b of the tube sheet. How this result is brought about will now be described.

A disklike baffle plate 13, having therein a plurality of orifices 14 one for each tube 1 l is mounted inside shell 6, at a location immediately adjacent to the outlet conduit 3, and specifically immediately adjacent the upper edge of this conduit. The baffle 13 thus provides one boundary for the space previously mentioned to wit, the space (in shell 6) between the outlet conduit or nozzle 3 and the backside 10b of the tube sheet. The bafl'le I3 is supported in the shell 6 by means of tube bundle tie rods (not shown), using spacer sleeves for location, in exactly the same manner as other tube bundle baffles (which are quite commonly used in conventional heat exchangers of the shell-and-tube and-tube type) are supported.

The baffle 13 substantially closes off the space between the outlet conduit 3 and the backside 10b of the tube sheet, converting this space into a chamber 15. Steam is injected into the chamber 15, at a pressure in excess of the pressure of the fluid within shell 6, thus providing a steam chamber. The steam is injected into chamber 15 by means of two injection nozzles or pipes 16, spaces l80around the shell 6, in a horizontal plane. The size of the tube clearances provided by the orifices l4, and the shell clearance provided by the CD. of the baftle 13, are made such as to flow a minimum amount of steam around each tube (through the orifices l4) and through the baffle shell clearance, while maintaining the pressure in chamber 15 slightly above or in excess of the pressure in the shall fluid, at the outlet conduit 3. The steam-filled chamber 15, at a pressure slightly above the shell side pressure in exchanger 1, provides a masking blanket of steam which prevents the hydrocarbon shell fluid (hot feed) from entering chamber 15 and thus from coming into contact with the backside 10b of the tube sheet 10; the hydrocarbon material is thereby kept out of the crevices 18 around the tubes 11, and coking is prevented.

As described, there is some leakage of steam from the chamber 15 into the shell fluid (EB feed), but this is inconsequential insofar as the process is concerned.

Although two steam injection nozzles have been referred to hereinabove, a greater number of nozzles could be used. The number of steam injection nozzles is not critical, but should be increased as the tube bundle diameter is increased, to maintain good uniformity in the steam chamber pressure profile. Likewise, the distribution of the nozzles should be as symmetrical as possible, for the same reason.

Fairly uniform steam distribution in the chamber 15 should be maintained, to create a uniformly higher (positive) pressure in the chamber 15 than in the shell fluid, at the outlet conduit 3. This positive pressure in chamber 15 is dependent upon the leakage clearances between the CD. of baffle 13 p The invention claimed is: 1. In a shell-and-tube heat exchanger: a tube sheet of appreciable thickness located at one end of the shell and within the same, a plurality of tubes mounted in individual holes in said tube sheet, each tube being sealed into said tube sheet at only one side of the sheet to leave a crevice around each tube at the other side of the sheet; an outlet conduit for the fluid flowing through the shell, said conduit being located adjacent to said other side of said tube sheet but spaced therefrom; and means providing a masking blanket of an inorganic vapor in the space between said conduit and said tube sheet and in said crevices, thereby to prevent said fluid from coming into contact with said tube sheet at said crevices.

2. Structure of claim 1, wherein the longitudinal axis of said shell extends vertically, and wherein said fluid absorbs heat upon flowing through said shell to said outlet conduit.

3. Structure of claim 1, wherein the pressure of said inorganic vapor blanket is in excess of the pressure of said fluid.

4. Structure of claim 1, wherein said fluid is a hydrocarbon, and wherein such hydrocarbon absorbs heat upon flowing through said shell to said outlet conduit.

5. Structure according to claim 4, wherein the longitudinal axis of said shell extends vertically.

6. Structure according to claim 4, wherein the pressure of said inorganic vapor blanket is in excess of the pressure of said hydrocarbon fluid.

7. Structure as defined in claim 1, including also a baffle plate within said shell and immediately adjacent to said outlet conduit, said bafile plate providing one boundary for said space.

8. Structure according to claim 7, wherein said baffle plate is positioned substantially horizontally within said shell, and is provided with apertures through which said tubes extend.

9. Structure set forth in claim 8, wherein said apertures are sized for a minimum flow of inorganic vapor therethrough,

thereby to permit maintaining the pressure of said blanket of inorganic vapor in excess of the pressure of said fluid.

10. Structure set forth in claim 8, wherein the longitudinal axis of said shell extends vertically, and wherein said fluid absorbs heat upon flowing through said shell to said outlet conduit.

11. Structure according to claim 7, wherein said fluid is a hydrocarbon, and wherein such hydrocarbon absorbs heat upon flowing through said shell to said outlet conduit. 

1. In a shell-and-tube heat exchanger: a tube sheet of appreciable thickness located at one end of the shell and within the same, a plurality of tubes mounted in individual holes in said tube sheet, each tube being sealed into said tube sheet at only one side of the sheet to leave a crevice around each tube at the other side of the sheet; an outlet conduit for the fluid flowing through the shell, said conduit being located adjacent to said other side of said tube sheet but spaced therefrom; and means providing a masking blanket of an inorganic vapor in the space between said conduit and said tube sheet and in said crevices, thereby to prevent said fluid from coming into contact with said tube sheet at said crevices.
 2. Structure of claim 1, wherein the longitudinal axis of said shell extends vertically, and wherein said fluid absorbs heat upon flowing through said shell to said outlet conduit.
 3. Structure of claim 1, wherein the pressure of said inorganic vapor blanket is in excess of the pressure of said fluid.
 4. Structure of claim 1, wherein said fluid is a hydrocarbon, and wherein such hydrocarbon absorbs heat upon flowing through said shell to said outlet conduit.
 5. Structure according to claim 4, wherein the longitudinal axis of said shell extends vertically.
 6. Structure according to claim 4, wherein the pressure of said inorganic vapor blanket is in excess of the pressure of said hydrocarbon fluid.
 7. Structure as defined in claim 1, including also a baffle plate within said shell and immediately adjacent to said outlet conduit, said baffle plate providing one boundary for said space.
 8. Structure according to claim 7, wherein said baffle plate is positioned substantially horizontally within said shell, and is provided with apertures through which said tubes extend.
 9. Structure set forth in claim 8, wherein said apertures are sized for a minimum flow of inorganic vapor therethrough, thereby to permit maintaining the pressure of said blanket of inorganic vapor in excess of the pressure of said fluid.
 10. Structure set forth in claim 8, wherein the longitudinal axis of said shell extends vertically, and wherein said fluid absorbs heat uPon flowing through said shell to said outlet conduit.
 11. Structure according to claim 7, wherein said fluid is a hydrocarbon, and wherein such hydrocarbon absorbs heat upon flowing through said shell to said outlet conduit. 